Dynamic Response of Intraplate Seismicity to the 2011 Tohoku-Oki Earthquake

Abstract:

Great earthquakes with magnitudes equal to or greater than 9.0 rarely occur over the globe. The influence of great earthquake on intraplate regions in regional distances has been poorly known. The Korean Peninsula belongs to intraplate regime with low seismicity, and is located in the far-eastern Eurasia plate. The instrumental seismicity present diffused seismicity with relatively high population around paleo-tectonic regions. The Korean Peninsula is placed at about 1300 km away from the 2011 M9.0 Tohoku-Oki earthquake. Earthquakes triggered by the Tohoku-Oki megathrust are searched from continuous waveform data using an automated event detection algorithm. The detection algorithm is to find the optimum event location where inner product of theoretical and observed STA/LTA of the waveform is maximized at hypothetical origin time. Events are declared when the inner product is greater than an empirically determined threshold. The detected events are approved after manual examination of signals. Sixty-one earthquakes with magnitudes of 0.5-2.5 are detected in the Korean Peninsula within 18 hours after the megathrust, including 17 clustered earthquakes in northwestern peninsula. The spatial distribution of clustered events suggests $\sim$279$^\circ$-directional strike-slip faulting. This fault geometry comprises a situation in which the great-circle direction is approximately subparallel with the fault strike. Most events occurred within 5 hours after the mainshock, yielding the p-value of $\sim$1.0 in Omori's law. The elevated Gutenberg-Richter (GR) a value suggests the increase of seismicity after the megathrust. The spatial distribution of seismicity, GR b values, and focal mechanisms solutions of triggered events are consistent with those of natural earthquakes before the megathrust. The observations suggest that only the occurrence rates were increased for dynamic stress change inducing increase of pore pressure by fluid transfer or fault weakening by fault plane alteration. An exceptionally large frictional rate change is observed, suggesting the influence of lithospheric dislocations and strong passage of transient waves.